Alumina powder and method for preparing the same as well as use thereof

ABSTRACT

Herein disclosed is alumina powder incorporated into a composition which should have excellent heat conduction and used as a heat-radiating member and for sealing a semiconductor. The spherical α-alumina powder has an average sphericity of not less than 0.93 and a content of α-crystalline form is not less than 95% and the spherical α-alumina powder is prepared according to the method, which comprises the steps of: (1) softening metallic aluminum powder or alumina powder through the treatment with a flame; (2) solidifying the softened powder by passing the same through a zone maintained at a temperature ranging from 800 to 500° C.; (3) increasing the content of α-phase by passing the solidified powder through a zone maintained at a temperature ranging from 950 to 1,500° C.; and (4) collecting the resulting powdery product while cooling the same.

TECHNICAL FIELD

The present invention relates to alumina powder, a method for thepreparation thereof and the use thereof.

BACKGROUND ART

Recently, the amount of heat generated by, for instance IC devicesincreases steadily as the progression of the miniaturization and speedupof electronic machinery and tools and accordingly, it has been requiredfor a heat-radiating member used around the surroundings ofheat-generating parts of the devices to have further improvedheat-radiation characteristics. In response to this, there has beenunder study the use of a resin or rubber filled with alumina powder asone of such heat-radiating members.

Alumina is present in a variety of crystalline states such as α, β, δ, γand θ-crystalline states, but α-alumina can suitably be used as amaterial for a heat-radiating member among others, since it has thehighest heat conduction. However, α-alumina powder is in general in theforms free of any crushed shapes or cut edges (or in the form ofedge-free shapes or round shapes). For this reason, in the existingcircumstances, it cannot be incorporated into resins or rubber at a highdensity or in a large amount and thus one cannot make the most use ofthe high heat conduction of α-alumina. In addition, a problem alsoarises such that these α-alumina powdery products may severely be wornout, for instance, the machinery and tools used in the blending thereofwith resins or the like such as kneaders and rolls and pumps used fortransporting a composition including, for instance, a resin as well as amold or the like used for molding the composition.

For this reason, there have variously been investigated the use ofspherical alumina powder.

In the meantime, spherical alumina powder is prepared according to atechnique fundamentally comprising a step of melting a raw material ofalumina through heating the same with a flame and the resulting aluminapowder is in crystalline states such as δ, θ, γ, and β-crystallinestate, which are inferior in the heat conduction to α-state. Forinstance, Patent Document 1 (specified below) discloses a method forpreparing spherical α-alumina powder, but the results of thesupplementary examinations of the method carried out by the inventors ofthis invention indicate that the content of α-alumina in the resultingproduct is 85% and a problem thus arises such that the method providesonly a product having an average sphericity on the order of at highest0.91 as expressed in terms of the evaluated value of the sphericity aswill be described later. In addition, Patent Document 2 (specifiedbelow) discloses a method for preparing pseudo-spherical particlesthrough the agglomeration of fine α-alumina particles, but the method ofthis literature simply provides, in the existing state, α-alumina powderwhose average sphericity is at highest about 0.80.

Patent Document 1: JP-A-2001-019425;

Patent Document 2: JP-A-9-086924.

DISCLOSURE OF THE INVENTION Subject to be Attained by the Invention

Accordingly, it is an object of the present invention to provideα-alumina powder whose average sphericity is not less than 0.93 and inwhich the content of α-crystalline form is not less than 95%. It isanother object of the present invention to provide a method forpreparing α-alumina powder having such characteristic properties as wellas a composition obtained by incorporating the resulting α-aluminapowder into, for instance, a resin.

More specifically, the following are herein provided according to thepresent invention:

1. α-Alumina powder characterized in that it has an average sphericityof not less than 0.93 and it has a content of α-crystalline form is notless than 95%.2. A method for preparing α-alumina powder which has an averagesphericity of not less than 0.93 and a content of α-crystalline form isnot less than 95%, characterized in that the method comprises the stepsof:(1) softening metallic aluminum powder or alumina powder through thetreatment with a flame;(2) solidifying the softened powder by passing the same through a regionmaintained at a temperature ranging from 800 to 500° C.;(3) increasing the content of α-phase by passing the solidified powderthrough a region maintained at a temperature ranging from 950 to 1,500°C.; and(4) collecting the resulting powdery product while cooling the same.3. A heat conductive composition comprising a resin or rubber, whichcontains α-alumina powder having an average sphericity of not less than0.93 and a content of α-crystalline form of not less than 95%,incorporated into the resin or the rubber.

Means for Attaining the Subject

The inventors of this invention have conducted intensive studies toobtain α-alumina powder having a very high sphericity and a high contentof α-crystalline form, have thus found that spherical α-alumina powderhaving an enhanced content of α-crystalline form can be prepared by amethod which comprises the steps of heat-treating metallic aluminumpowder or alumina powder by applying a flame thereto to thus soften thesame, guiding it to a collection system while cooling the same to thusrecover alumina powder, wherein the alumina powdery product heat-treatedwith a flame is once passed through a region or zone maintained at atemperature ranging from 800 to 500° C. to thus solidify theheat-treated powdery product and further passed through a region or zonemaintained at a higher temperature ranging from 950 to 1,500° C. andhave thus completed the present invention.

EFFECTS OF THE INVENTION

The present invention permits the production of spherical α-aluminapowder having an average sphericity of not less than 0.93 and a contentof α-crystalline form or a rate of α-phase (crystalline form) of notless than 95%. Moreover, if the α-alumina powder having suchcharacteristic properties is incorporated into a resin or rubber, theresulting blend containing the alumina powder may have a high aluminadensity. Moreover, the α-alumina powder of the present invention has ahigh content of α-crystalline form and this accordingly results in theproduction of, for instance, a resin composition which is excellent inthe heat conduction and which is thus quite useful as a heat-radiatingmaterial used in the production of, for instance, IC devices.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention will be described in more detail below.

The α-alumina powder according to the present invention has suchcharacteristic properties as an average sphericity of not less than 0.93and a content of α-crystalline form on the order of not less than 95%.

If the α-alumina powder of the present invention is incorporated into,for instance, a resin or rubber, the resulting composition does not havean extremely high viscosity, the powder can be incorporated into, forinstance, a resin in a large amount and the powder is likewise excellentin the flow properties, since the powder has an average sphericity ofnot less than 0.93. Accordingly, the α-alumina powder of the presentinvention is quite useful when molding, for instance, a resin containingα-alumina powder in a mold.

The average sphericity of the α-alumina powder of the present inventionis preferably equal to or higher than 0.95. Particularly preferred isone having an average sphericity of almost equal to the upper limit or1.00. From the practical stand point, it may be possible to prepareα-alumina particles having an average sphericity very close to that ofthe complete sphere. In this connection, the higher the averagesphericity of the α-alumina particles, the higher the flow propertiesthereof and the risk of causing any abrasive wear of a variety ofmachinery and tools would be substantially reduced.

In this regard, the average sphericity of the α-alumina powder of thepresent invention is determined and defined according to the followingmethods:

Micrographs of alumina particles are taken using a scanning electronmicroscope (Model JXA-8600M available from JEOL Ltd.) at a magnificationof ×500 for alumina particles having a particle size of not less than 30μm, ×3000 for alumina particles having a particle size of not less than5 μm and less than 30 μm, ×5000 for alumina particles having a particlesize of not less than 1 μm and less than 5 μm, and ×50000 for aluminaparticles having a particle size of less than 1 μm, the projected area(A) and the peripheral length (perimeter: PM) of the particles aredetermined on the basis of the secondary electron-reflected images (SEMmicrograph images) and these data are substituted in the followingequation to thus determine the desired average sphericity.

In other words, if the area of the true circle corresponding to theperimeter (PM) of a particle is assumed to be (B), the sphericity of theparticle can be represented by the following relation: A/B. In thisrespect, if assuming a true circle having a peripheral length identicalto the perimeter (PM) of a sample particle, parameters PM and B can beexpressed as follows: PM=2πr and B=πr², and therefore, the followingrelation holds: B=π×(PM/2π)². Consequently, the sphericity of thisparticle is as follows: Sphericity=A/B=A×4 π/(PM)². Thus, the sphericityis determined for 100 particles arbitrarily selected from the SEMmicrograph images and the average of these measurements is defined to bethe average sphericity.

If the content of the α-crystalline form in alumina powder is less than95%, the resulting composition of, for instance, a resin may onlyslightly be improved in its heat conduction even if such α-aluminapowder can be incorporated into the composition in a high density andtherefore, the composition is insufficient for use as a heat-radiatingmaterial having high heat-radiation properties. The most preferred isα-alumina powder having a content of α-crystalline form almost equal tothe upper limit thereof or an α-alumina content of 100%.

The α-alumina powder used herein is preferably one free of anycrystalline phase other than the α-phase, but it would include, forinstance, δ-alumina and θ-alumina as inevitable components in an amountof at highest 5% and even if such inevitable components are mixed in theα-alumina powder used herein, this never causes any problem in thepractice of the present invention at all.

The rate of the α-alumina phase is determined in a powder X-raydiffraction device using Cu—K a rays according to the following method:The measurement was carried out using a powder X-ray diffractometer (forinstance, “JDX-3500” available from JEOL Ltd.) and a scintillationcounter as a detector under the following conditions of measurement: anapplied voltage of 40 kV; an electric current of 300 mA; diverging slit:1 deg.; scattering slit: 1 deg.; light-receiving slit: 0.2 mm. Themeasurement was carried out at a step angle of 0.02 deg./step and ameasuring time of 0.5 sec/step according to the 2 θ·θ scanningtechnique. In addition, the measuring range was set at a level of 2 θ=30to 50 degs. In the first place, the calibration curve is prepared.Samples herein used for preparing such a calibration curve wereα-alumina (available from Kanto Chemical Co., Ltd. under the trade nameof “Aluminum Oxide (α-type)) and δ-alumina (available from Denki KagakuKogyo K.K. under the trade name of “ASFP-20”). The calibration curve wasprepared through the use of 11 sample powders obtained by blendingα-alumina and δ-alumina in mixing ratios of 0:100, 1:99, 3:97, 5:95,7:93, 10:90, 20:80, 50:50, 75:25, 90:10 and 0:100 and plotting theresults on the X-Y coordinates, with the peak area of the (113) face onthe X-axis and the rate of α-alumina on the Y-axis. Then, each samplepowder was inspected for the peak area (Y) of the (113) face andsubstituted in the following equation: Rate of α-Alumina (% bymass)=[Y−(the intercept of the calibration curve)]/(the gradient of thecalibration curve) to thus obtain the rate of α-alumina.

The particle size of spherical α-alumina powder may vary depending onits use and the method for using the same. For instance, when it is usedas a heat-radiating material, the particle size thereof is up to thelevel corresponding to the thickness of the heat-radiating material andit ranges, for instance, from 0.01 to 50 μm in case where the thicknessof the heat-radiating material is 0.1 mm, while if it is used in anepoxy resin composition as an IC device-sealing material, the particlesize preferably ranges from 0.01 to 100 μm.

Then a preferred embodiment of the method for preparing sphericalα-alumina powder according to the present invention will be describedbelow in more detail with reference to the accompanying drawings.

FIG. 1 shows a device preferably used for the production of thespherical α-alumina powder according to the present invention. A heatingdevice (6) for re-heating solidified powder body is arranged below afurnace (5) for forming a flame and a collecting device (9) forcollecting the resulting spherical α-alumina powder is likewisepositioned below the heating device. On the other hand, a burner (2) forforming a flame and a raw material-supply port (1) are positioned abovethe furnace (5). The furnace (5) may be either a vertical or horizontaltype one.

The raw powder used herein may be metallic aluminum powder, aluminapowder or a mixed powder thereof. The use of metallic aluminum powderwould permit the production of ultrafine powdery product. The use ofalumina powder, for instance, raw alumina powder having a particle sizeof 50 μm would permit the production of spherical α-alumina powder whoseparticle size is on the order of 50 μm. The raw powder may be suppliedto the furnace in its dried powdery form or may be supplied theretoafter it is converted into a slurry in a medium such as an alcohol orwater. In the present invention, it is preferably supplied to thefurnace while it is accompanied by or entrained with a carrier gas suchas oxygen or air.

The flame can be formed by combusting a combustion gas such as hydrogengas, natural gas, acetylene gas, propane gas and/or butane gas injectedto the furnace through a combustion gas-supply port (3) while supplyinga combustion, improving gas such as air or oxygen gas through acombustion-improving gas-supply port (4). It is herein suitable that theflame temperature is set at a level of, for instance, not less than1800° C. and preferably not less than 2100° C. The upper limit of theflame temperature may be increased to a level of, for instance, up to2500° C. The raw powder subjected to such a heat-treatment with a flameis softened and melted, is then passed through a zone maintained at atemperature ranging from 500 to 800° C. and preferably 680 to 780° C. tothus solidify the molten raw material into spherical alumina powder andthen the solidified raw material is further introduced into a heatingdevice (6). In this respect, it would be quite important in the presentinvention that the product heat-treated with a flame is once passedthrough a zone maintained at a temperature ranging from 500 to 800° C.and then passed through a zone maintained at a temperature ranging from950 to 1500° C. and preferably 1050 to 1500° C. or more specificallysubjected to a re-heating treatment in a heating device maintained atthat temperature.

In the present invention, the temperature prior to the re-heatingtreatment is set at a level of 500 to 800° C. for the following reason:if the softened or melted raw material is introduced into the heatingdevice (6), while the material is still maintained at a temperaturehigher than 800° C., the heat-treated product is insufficientlysolidified and this accordingly never results in the recovery of aluminapowder whose average sphericity is not less than 0.93. While if thetemperature prior to the re-heating treatment is less than 500° C., theδ-crystalline phase and θ-crystalline phase of the resulting aluminapowder are stabilized, this leads to the occurrence of a shape change ofthe alumina powder during the re-heating step and this also neverresults in the formation of alumina powder having an average sphericityof not less than 0.93. It is preferred in the present invention that theresidence time of the heat-treated product in the zone having atemperature ranging from 500 to 800° C. is not less than 1.0×10⁻³ secand preferably not less than 0.1 sec. This residence time can becontrolled by appropriately adjusting the gas-flow rate in the furnace.

The atmosphere in the heating device (6) is maintained at a temperatureranging from 950 to 1500° C. This temperature control is carried out byany external heating means, for instance, gas combustion or heating withan electric heater through the furnace wall. In the apparatus as shownin FIG. 1, the temperature control is carried out by the gas combustionsystem using gases supplied through a combustion gas-supply port (7) anda combustion-improving gas-supply port (8). It is preferred in thepresent invention that supply ports for the mixed gas comprising acombustion gas and a combustion-improving gas are uniformly arranged ina number as great as possible to prevent any local heating. In thisrespect, if the atmospheric temperature of the heating device is lessthan 950° C., the alumina powder does not easily undergo thephase-transition into its α-phase, while if it exceeds 1500° C.,particles may be fused and combined together to thus adversely affectthe average sphericity of the resulting alumina powder. It is preferredin the invention to set, at a level of not less than 1.0 sec, theresidence time of the solidified alumina powder in the zone maintainedat a temperature ranging from 950 to 1500° C. This residence time can becontrolled by the gas-flow rate in the furnace.

The spherical α-alumina powder passed through the heating device (6) isrecovered by a collecting device (9) and only the exhaust gas isdischarged through, for instance, a blower (10). Such a collectingdevice usable herein includes, for instance, a gravity-settling chamber,a cyclone which makes use of a centrifugal separator or a bug filter.

The spherical α-alumina powder of the present invention is incorporatedinto, for instance, a resin to thus give a composition which can be usedin a variety of applications.

Examples of rubber materials suitably used herein include siliconerubber, urethane rubber, acrylic rubber, butyl rubber,ethylene-propylene rubber, urethane rubber, and ethylene-vinyl acetatecopolymer. On the other hand, examples of resins suitably used hereininclude epoxy resin, silicone resin, phenol resin, melamine resin, urearesin, unsaturated polyester, fluorine atom-containing resin, polyimide,polyamide such as polyamide-imide and polyether-imide, polyesters suchas polybutylene terephthalate and polyethylene terephthalate,polyphenylene sulfide, wholly aromatic polyester, polysulfones, liquidcrystalline polymer, polyether sulfone, polycarbonate,maleimide-modified resin, ABS resin, AAS (acrylonitrile-acrylicrubber/styrene) resin, and AES (acrylonitrile/ethylene/propylene/dienerubber-styrene) resin.

It is suitable that the alumina powder according to the presentinvention is preferably incorporated into, for instance, a resin in anamount of, for instance, 50 to 95% by mass and, in particular, 70 to 93%by mass.

Among these resins or the like, preferably used in the field of theheat-radiating material are, for instance, silicone resins whoseorgano-polysiloxane backbone is composed of dimethylsiloxane units andsilicone resins whose organo-polysiloxane backbone comprises, forinstance, vinyl groups, phenyl groups, and trifluoro-propyl groupsincorporated therein. Moreover, if the heat-radiating material is highlyflexible one having an ASKER C hardness of less than 25, preferably usedherein are addition reaction type liquid silicone rubber such as thoseobtained through addition reactions of addition-reactive type liquidsilicone rubber such as a one-pack type addition reactive siliconehaving both vinyl and H—Si groups in a molecule, or through additionreactions of two-pack type addition reaction type silicone comprisingorgano-polysiloxane carrying vinyl groups at terminals or on side chains(liquid A) and organo-polysiloxane carrying at least two H—Si groups atterminals or on side chains (liquid B).

The base polymer used herein and constituting such a one-pack typeaddition reactive silicone or a two-pack type addition reactive siliconehas, in its backbone, organic groups such as methyl, phenyl andtrifluoro-propyl groups. It is preferred that the mixing ratio ofaddition reactive vinyl groups and H—Si groups ranges from 0.5 to 3 moleequivalents and preferably 1 to 2 mole equivalents of H—Si group per onemole of vinyl group from the viewpoint of the curing speed of theresulting rubber and the physical properties of the rubber after thecuring of the same. Moreover, in the addition reaction of the additionreactive liquid silicone rubber, a catalyst for the addition reactioncan be used for the acceleration of the addition reaction and specificexamples thereof include platinum atom-containing catalysts such as Pt,platinum black, chloroplatinic acid, alcohol-modified chloroplatinicacid, and a complex of chloroplatinic acid and an olefin.

Specific examples of such addition reactive liquid silicone rubberinclude “TSE3070” and “TSE3051” available from Toshiba Silicone Co.,Ltd. or “SE1880”, “SE1885A/B”, “SE1886A/B”, “SE1887A/B”, “SE4440A/B”,“SE1891KA/B”, and “CY52-283A/B” available from Toray Silicone Co., Ltd.

When using the composition in the form of a resin composition forsealing semiconductors, preferably used include epoxy resins eachcarrying at least two epoxy groups in a molecule. Specific examplesthereof include phenol-novolak type epoxy resins, o-cresol-novolak typeepoxy resins, those obtained by converting novolak resins consisting ofphenols and aldehydes into epoxy derivatives, glycidyl ethers such asbisphenol A, bisphenol F and bisphenol S, glycidyl ester acid epoxyresins obtained through the reaction of polybasic acids such as phthalicacid and dimer acid with epichlorohydrin, linear aliphatic epoxy resins,alicyclic epoxy resins, heterocyclic epoxy resins, alkyl-modifiedpolyfunctional epoxy resins, β-naphthol-novolak type epoxy resins,1,6-dihydroxynaphthalene type epoxy resins, 2,7-dihydroxynaphthalenetype epoxy resins, bishydroxy-biphenyl type epoxy resins, and furtherepoxy resins into which halogen atoms such as bromine atoms areintroduced to thus impart flame retardancy to the resins. Among them,preferably used herein include, for instance, o-cresol-novolak typeepoxy resins, bishydroxy-biphenyl type epoxy resins, and epoxy resinshaving naphthalene skeletons, from the viewpoint of themoisture-resistant properties and the solder-reflow resistantproperties.

The curing agent for curing the epoxy resin is not restricted to anyparticular one insofar as it can react with the epoxy resin to thus curethe same and examples thereof include novolak type resins obtained byreacting at least one member selected from the group consisting of, forinstance, phenol, cresol, xylenol, resorcinol, chlorophenol,t-butylphenol, nonylphenol, isopropyl-phenol, and octylphenol withformaldehyde, para-formaldehyde or p-xylene in the presence of anoxidation catalyst, poly(p-hydroxystyrene) resins, bisphenol compoundssuch as bisphenol A and bisphenol S, trifunctional phenols such aspyrogallol and fluoroglucinol, acid anhydrides such as maleic anhydride,phthalic anhydride and pyromellitic anhydride, and aromatic amines suchas m-phenylene-diamine, diaminodiphenyl-methane anddiaminodiphenyl-sulfone.

A curing accelerator may be used for the acceleration of the reaction ofepoxy resins and curing agents for curing the same. As such curingaccelerators, there may be listed, for instance, 1,8-diazabicyclo(5,4,0)undecene-7, triphenyl phosphine, benzyldimethyl-amine, and2-methylimidazole.

The following components may, if necessary, be incorporated into thecomposition containing the spherical α-alumina powder of the presentinvention. More specifically, a silane-coupling agent can be used andexamples thereof include epoxy silanes such asγ-glycidoxypropyl-trimethoxysilane andβ-(3,4-epoxy-cyclohexyl)ethyl-trimethoxy-silane; aminosilanes such asaminopropyl-triethoxysilane, ureidopropyl-triethoxysilane andN-phenylaminopropyl-trimethoxysilane; hydrophobic silane compounds suchas phenyltrimethoxy-silane, methyltrimethoxy-silane, andoctadecyltrimethoxy-silane; mercapto-silane; surface-treating agentssuch as Zr-chelates, titanate coupling agents and aluminumatom-containing coupling agents; flame-retardant improvers such asSb₂O₃, Sb₂O₄, and Sb₂O₅; flame-retardant agents such as halogenatedepoxy resins and phosphorus atom-containing compounds; and coloringagents such as carbon black, iron oxide, dyes and pigments. Moreover, areleasing agent such as a wax may likewise be added to the compositionand specific examples thereof are naturally occurring waxes, syntheticwaxes, metal salts of linear fatty acids, acid amides, esters, andparaffin. In particular, when the composition is required to have highlyreliable moisture resistance and high stability when the composition isallowed to stand at a high temperature, it is effective to add a varietyof ion-trapping agents. Examples of commercially available ion-trappingagents include those sold by Kyowa Chemical Co., Ltd. under the tradename of DHF-4A, KW-2000, KW-2100, and those sold by Toagosei ChemicalIndustry Co., Ltd. under the trade name of “IXE-600”.

EXAMPLES

Now, the present invention will be described in more detail withreference to, for instance, the following Examples and ComparativeExamples, but the scope of the present invention is not limited to, forinstance, these Examples and Comparative Examples at all.

Examples 1 to 5, Comparative Examples 1 to 6 and Reference Examples 1 to2

Spherical α-alumina powder products were prepared using the verticaldevice as shown in FIG. 1.

A heating device (6) having a size of 400 mm (diameter)×5000 mm (height)was connected to the lower end of a furnace (5) having a size of 500 mm(diameter)×2000 mm (height). The heating device (6) is provided withcombustion gas (propane gas)-supply pipes (7) and combustion-improvinggas (oxygen gas)-supply pipes (8), which are branched into 20 pairs. Abag filter was used as a collecting device (9).

There were supplied, to the furnace, propane gas (LPG) through acombustion gas-supply port (3) of a burner (2) and oxygen gas, as acombustion-improving gas, through a combustion-improving gas-supply port(4) to thus form a flame (having a temperature of not less than 2000°C.) (the temperature was determined at the center of the flame and atthe distance, 200 mm, from the tip of the burner (2)). The formation ofthe flame was carried out by supplying LPG at a discharge rate of notless than 20 m/sec through a combustion gas-supply port (a slitthickness of 1 mm) arranged at the outer periphery of the dischargeopening of a raw material-supply port (1) connected to the center of theburner at the tip thereof; and further supplying oxygen gas at adischarge rate of not less than 5 m/sec through a combustion-improvinggas-supply port (a slit thickness of 10 mm) arranged at the outerperiphery of the discharge opening of a combustion gas-supply port. Eachpowdery raw material in the form of its dried powder and detailed in thefollowing Table 1 was fed, at a flow rate of 30 kg/hr, to the furnacethrough the raw material-supply port (1) while the powder was entrainedwith oxygen gas in a flow rate of 25 Nm³/hr. The atmospherictemperatures of a zone maintained at a temperature ranging from 800 to500° C. and a zone maintained at a temperature ranging from 950 to 1500°C. were determined at the following three points: point of measurement A(situating at the outlet of the furnace) (the position situating at thedistance of 4900 mm from the upper face of the furnace (5) in FIG. 1);point of measurement B (situating at the inlet of the heating device (6)and more specifically, at the boundary between the furnace (5) and theheating device (6)); and point of measurement C (situating at the outletof the heating device (6)). There were used, in these measurements, acommercially available K-thermocouple within the temperature range offrom 0 to 600° C. and a commercially available B-thermocouple within thetemperature range of from 600 to 1700° C. (both of them are availablefrom Chino Corporation).

The conditions for the heat-treatment are summarized in the followingTable 2. The following Table 3 shows characteristic properties of thealumina powdery products recovered from the bag filter. In thisconnection, Table 3 also shows the characteristic properties of crushedalumina “AS-50” commercially available from Showa Denko, K.K. (averageparticle size: 10 μm) as Reference Example 1 and crushed alumina “AA-05”commercially available from Sumitomo Chemical Co., Ltd. (averageparticle size: 0.6 μm) as Reference Example 2.

TABLE 1 Raw Powder 1 Amorphous alumina powder (average particle size: 50μm) Raw Powder 2 Amorphous alumina powder (average particle size: 3 μm)Raw Powder 3 Metal aluminum powder (average particle size: 10 μm)

TABLE 2 Amt. supplied to Re-heating device Raw furnace (Nm³/hr) (Nm³/hr)Powder LPG Oxygen gas LPG Oxygen gas Ex. 1 1 10 50 5 25 Ex. 2 1 15 75 525 Ex. 3 1 10 50 3.5 10.5 Ex. 4 2 10 50 5 25 Ex. 5 3 3 15 5 25 Comp. Ex.1 1 20 100 3 15 Comp. Ex. 2 1 7 35 5 25 Comp. Ex. 3 1 10 50 2 10 Comp.Ex. 4 1 15 75 8 40 Comp. Ex. 5 2 10 50 0 0 Comp. Ex. 6 3 3 15 0 0Temperature at point of measurement (° C.) Residence time (sec) A B C550-900° C. 950-1500° C. Ex. 1 552 1348 1047 1.1 3.6 Ex. 2 748 1451 11030.7 2.7 Ex. 3 550 1101 987 1.1 4.5 Ex. 4 558 1346 1047 1.2 3.7 Ex. 5 7501462 1048 3.5 6.1 Comp. Ex. 1 909 1448 1105 0.0 2.3 Comp. Ex. 2 432 1092980 1.4 5.0 Comp. Ex. 3 553 879 680 1.2 0.0 Comp. Ex. 4 746 1647 15390.7 0.0 Comp. Ex. 5 555 501 296 1.3 0.0 Comp. Ex. 6 730 680 350 3.3 0.0

TABLE 3 Average Particle Average Sphericity Rate of α-Phase Size (μm)(—) (%) Ex. 1 51 0.94 98.2 Ex. 2 52 0.96 100.0 Ex. 3 51 0.95 96.3 Ex. 411 0.98 97.1 Ex. 5 0.6 0.98 96.5 Comp. Ex. 1 55 0.88 100.0 Comp. Ex. 251 0.87 98.0 Comp. Ex. 3 51 0.94 79.8 Comp. Ex. 4 52 0.89 100.0 Comp.Ex. 5 11 0.98 28.0 Comp. Ex. 6 0.6 0.98 Not less than 1.0 Ref. Ex. 1 100.85 100.0 Ref. Ex. 2 0.6 0.87 100.0

According to Examples 1 to 5, there were prepared alumina powderyproducts each having a high average sphericity and a high rate ofα-phase as compared with those observed for the alumina powdery productsprepared in Comparative Examples 1 to 6.

Then the alumina powdery products prepared in Examples 1 to 4 andComparative Examples 1 to 4 were blended with resins to prepare resincompositions and the resulting compositions were inspected for thefollowing physical properties, according to the following methods.

An epoxy resin, a curing agent, a curing-accelerator, a releasing agentand a silane-coupling agent were blended together in a mixing ratiospecified in the following Table 4, subsequently each resulting blendwas admixed with the foregoing alumina powder at a charging rate of 65%by mass and then each resulting blend was kneaded with heating in anormal mating type twin-screw extrusion-kneader (screw diameter, D: 25mm; kneading disk length: 10D mm; rotational number of paddle: 150 rpm;discharge rate: 4.5 kg/hr; heater temperature: 105 to 110° C.). Thedischarged product was cooled in a cooling press, then pulverized togive each composition and the resulting composition was inspected forthe heat conductivity, spiral flow and amount of abrasive wear of moldaccording to the following methods. The results thus obtained aresummarized in the following Table 5.

(1) Heat Conductivity:

A composition was poured into a mold provided with a hollow having adiameter of 28 mm and a thickness or depth of 3 mm, the composition wasthen degassed and molded at 150° C. for 20 minutes to give a molded bodyand the resulting molded body was inspected for the heat conductivityusing a heat conductivity-determining device (commercially availablefrom AGNE Company under the trade name of “ARC-TC-1 Model”) at roomtemperature according to the temperature-gradient technique to thusdetermine the heat conductivity of the molded body.

(2) Spiral Flow

The spiral flow was determined using a spiral flow mold according to themethod specified in EMMI-66 (Epoxy Molding Material Institute; Societyof Plastic Industry). In this measurement, the mold temperature was setat 175° C., the molding pressure was set at 7.4 MPa and thepressure-maintaining time was set at 90 seconds.

(3) Abrasive Wear of Mold

A composition which was heated to 175° C. was passed through a hole ofan aluminum disk, which had a thickness of 6 mm and a hole diameter of 3mm, in an amount of 150 cm³ using a pressure extruder and the weightloss of the disk was determined and the result thus obtained was definedto be the abrasive wear.

TABLE 4 Rate of Mixing Kind of Material (% by mass) Epoxy resino-Cresol/novolak type one 63.8 (available from Nippon Kayaku Co., Ltd.)Curing agent Phenol/novolak resin 32.1 (“PSM-4261” available from GuneiChemical Co., Ltd.) Curing Accelerator Triphenyl phosphine 0.6(available from Hokko Chemical Co., Ltd.) Releasing agent Montanic acidester 3.5 (“WaxEflakes” available from KURARI & Japan Co., Ltd.) Silanecoupling Organo-silane (“KBM403” 0.4 relative to agent available fromShin-Etsu alumina powder Chemical Co., Ltd.)

TABLE 5 Heat conductivity Abrasive wear of (W/mK) Spiral Flow (m) mold(mg) Ex. 1 4.3 1.1 2.2 Ex. 2 4.5 1.2 2.2 Ex. 3 4.2 1.1 2.1 Comp. Ex. 14.2 0.6 8.8 Comp. Ex. 2 4.1 0.7 6.9 Comp. Ex. 3 3.0 1.1 2.1 Comp. Ex. 44.3 0.8 7.1

Furthermore, resin compositions used for forming heat-radiating memberswere prepared through the use of the alumina powder products prepared inExample 4, Comparative Example 5 and Reference Example 1 (all of themhad an average particle size of 10 μl m or 11 μm); or the aluminapowdery products obtained in Example 5, Comparative Example 6 andReference Example 2 (all of them had an average particle size of 0.6 μm)according to the following procedures and then the resulting resincompositions were inspected for the physical properties likewiseaccording to the following methods.

Two-component addition reactive liquid silicone rubber products (“YE5822Liquid A” and “YE5822 Liquid B” available from GE Toshiba Silicone Co.,Ltd.), alumina powder and a retarder were blended in a mixing ratiospecified in the following Table 6 and the resulting compositions wereinspected for the following physical properties. The results obtainedusing the alumina powdery products prepared in Example 4, ComparativeExample 5 and Reference Example 1 are summarized in the following Table7, while the results obtained using the alumina powdery productsprepared in Example 5, Comparative Example 6 and Reference Example 2 aresummarized in the following Table 8.

(4) Heat Conductivity of Heat Radiating Member

To the silicone rubber liquid A, there were repeatedly added andstirred, in order, the retarder, the alumina powder and the siliconerubber liquid B and then the resulting mixture was degasses. Theresulting liquid sample was poured into a mold provided with a hollowhaving a diameter of 28 mm and a thickness or depth of 3 mm, the mixturewas then degassed and molded at 150° C. for 20 minutes to give a moldedbody and the resulting molded body was inspected for the heatconductivity according to the temperature-gradient technique to thusdetermine the heat conductivity of the molded body. In this respect,used herein as the measuring device was a heat conductivity-determiningdevice commercially available from AGNE Company under the trade name of“ARC-TC-1 Model.”

(5) Viscosity

The silicone rubber composition prepared above and prior to molding withheating was subjected to viscosity measurement using a B-type viscometer(“DB-10” available from DAIWA Kenko Co., Ltd.) at 30° C. and at arotational number of 20 rpm.

(6) Abrasive Wear of Mold

The silicone rubber composition prepared above and prior to molding withheating was passed through a hole of an aluminum disk, which had athickness of 6 mm and a hole diameter of 3 mm, in an amount of 150 cm³using an ordinary temperature-pressure extruder and the weight loss ofthe disk was determined and the result thus obtained was defined to bethe abrasive wear.

TABLE 6 Rate of Incorporation (% by mass) Ex. 4, Ex. 5, Comp. Comp. Ex.5, Ex. 5, Kind of Material Ref. Ex. 1 Ref. Ex. 1 Silicone rubber LiquidA; available from GE 18.2 24.5 Toshiba Silicone Co., Ltd. under thetrade name of “YE5822 Liquid A” Silicone rubber Liquid B; available fromGE 1.8 2.5 Toshiba Silicone Co., Ltd. under the trade name of “YE5822Liquid B” Alumina powder 80.0 73.0 Retarder: Dimethyl maleate, availablefrom 0.01 Kanto Chemical Co., Ltd. (Extra Pure (with respect to thetotal Reagent) amount of Liquid A + Liquid B)

TABLE 7 Heat Conductivity Abrasive Wear of (W/mK) Viscosity (mPa · s)Mold (mg) Ex. 4 2.8 140,000 1.5 Comp. Ex. 5 1.6 142,000 1.5 Ref. Ex. 1Not moldable Not measurable 6.5

TABLE 8 Heat Conductivity Abrasive Wear of (W/mK) Viscosity (mPa · s)Mold (mg) Ex. 5 1.9 176,000 0.10 Comp. Ex. 6 0.9 176,000 0.10 Ref. Ex. 2Not moldable Not measurable 0.25

As will be clear from the comparison of Examples with ComparativeExamples, the resin compositions and the heat-radiating members preparedusing the alumina powder according to the present invention were foundto be ones each having a high heat conductivity, excellent flowproperties and a low abrasive wear of mold.

INDUSTRIAL APPLICABILITY

The alumina powder according to the present invention can be used, forinstance, as a filler for a resin composition used for sealing asemiconductor and a filler for a heat-radiating member. Moreover, theheat-radiating member prepared using the spherical α-alumina powder ofthe present invention can be used, for instance, as a heat-radiatingsheet or a heat-radiating spacer when assembling electronic machineryand tools.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram for illustrating an embodiment of the device used inthe practice of the production method according to the presentinvention.

DESCRIPTION OF LETTERS OR NUMERALS

-   1 Raw material-supply port;-   2 Burner;-   3 Combustion gas-supply port;-   4 Combustion-improving gas-supply port;-   5 Furnace;-   6 Heating device;-   7 Combustion gas-supply pipe;-   8 Combustion-improving gas-supply pipe;-   9 Collecting device;-   10 Blower;-   A, B, C Point of measurement of temperature.

1. α-Alumina powder characterized in that it has an average sphericityof not less than 0.93 and it has a content of α-crystalline form is notless than 95%.
 2. The α-alumina powder as set forth in claim 1, whereinthe average sphericity is not less than 0.95.
 3. The α-alumina powder asset forth in claim 1, wherein the content of α-crystalline form is 100%.4. A method for preparing spherical α-alumina powder which has anaverage sphericity of not less than 0.93 and a content of α-crystallineform among others is not less than 95%, characterized in that the methodcomprises the steps of: (1) softening metallic aluminum powder oralumina powder through the treatment with a flame; (2) solidifying thesoftened powder by passing the same through a zone maintained at atemperature ranging from 800 to 500° C.; (3) increasing the content ofα-phase by passing the solidified powder through a zone maintained at atemperature ranging from 950 to 1,500° C.; and (4) collecting theresulting powdery product while cooling the same.
 5. The method as setforth in claim 4, wherein the softened powder is passed through a zonemaintained at a temperature ranging from 680 to 780° C. in the step (2).6. The method as set forth in claim 4, wherein the softened powder ispassed through a zone maintained at a temperature ranging from 1,050 to1,500° C. in the step (2).
 7. The method as set forth in claim 4,wherein the average sphericity is not less than 0.95.
 8. The method asset forth in claim 4, wherein the content of α-crystalline form is 100%.9. A heat conductive composition comprising a resin or rubber, whichcontains spherical α-alumina powder having an average sphericity of notless than 0.93 and a content of α-crystalline form of not less than 95%,incorporated into the resin or the rubber.
 10. The heat conductivecomposition as set forth in claim 9, wherein the average sphericity isnot less than 0.95.
 11. The heat conductive composition as set forth inclaim 9, wherein the content of α-crystalline form is 100%.
 12. The heatconductive composition as set forth in claim 9, wherein the sphericalα-alumina powder is incorporated into the composition in an amountranging from 50 to 95% by mass.
 13. The heat conductive composition asset forth in claim 9, wherein the spherical α-alumina powder isincorporated into the composition in an amount ranging from 70 to 93% bymass.
 14. The heat conductive composition as set forth in claim 9,wherein the resin is an epoxy resin.
 15. The heat conductive compositionas set forth in claim 14, wherein it is used for sealing asemiconductor.
 16. The heat conductive composition as set forth in claim9, wherein the rubber is silicone rubber.
 17. The heat conductivecomposition as set forth in claim 16, wherein it is used as aheat-radiating member.